Detachably coupled image intensifier and image sensor

ABSTRACT

A detachably coupled image intensifier and image sensor combination is disclosed along with systems and methods for using the detachably coupled image intensifier and image sensor combination. In one embodiment, there are at least two fiber optic plates aligned between the image intensifier and image sensor, and an oil or a gel is used to fill some or all of the gap(s) between pair(s) of adjacent fiber optic plates. In one embodiment, the detachably coupled image intensifier and image sensor combination is used for sample inspection.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 11/516,668filed Sep. 7, 2006, which is a continuation-in-part of U.S. Ser. No.10/511,092, filed Apr. 26, 2005; itself a national stage application ofPCT/US03/28062, filed Sep. 8, 2003; which claims the benefit of U.S.Provisional Patent Application 60/415,082, filed Sep. 30, 2002.

This application also claims the benefit of U.S. Provisional Application60/715,927, U.S. Provisional Patent Application 60/715,900, and U.S.Provisional Application 60/715,901. Said applications were all filed onSep. 8, 2005 and are hereby incorporated by reference herein.

FIELD OF THE INVENTION

This invention relates to image intensifier tubes.

BACKGROUND OF THE INVENTION

Image intensifier tubes (also known as IIT or image intensifiers) arewidely used for sensing and amplifying, or intensifying, light images oflow intensity. In these devices, light (usually of visible or nearinfra-red spectra) from an associated optical system is directed onto aphotocathode which emits a distribution of photoelectrons in response tothe input radiation.

An image intensifier typically includes a vacuum tube with aphotocathode unit at one end and a screen unit at the other end. Thephotocathode unit converts incoming photons to electrons which areaccelerated by an electric field (potential difference) in the tubeuntil they hit a screen unit converting them back to photons.

The output of the image intensifier tube is fed into a solid stateoptical image sensor such as a charge coupled device (CCD) or acomplementary metal-oxide semiconductor (CMOS) device. The combinationof the image intensifier tube and image sensor is sometimes referred toas an intensified image sensor device ICCD or ICMOS.

In U.S. Pat. No. 4,980,772 to Kawamura et al, some examples of currentmethods of coupling the image intensifier tube to an image pickup deviceare disclosed. In one method, a thin fiber plate is interposed between aphosphor layer of the screen of the image intensifier tube and aphotosensitive layer of the image pickup device. In another method, twofiber plates are used to couple an image intensifier tube to the imagepickup device. In yet another method, a fiber plate on the outputsurface of the image intensifier tube is bound to an image pickup deviceby means of an adhesive.

SUMMARY OF THE INVENTION

According to the present invention there is provided an apparatus forintensifying and sensing images, comprising: an image intensifier tube;an image sensor; at least two fiber optic plates aligned between aphoto-emitting output area of the image intensifier tube and aphotosensitive input area of the image sensor so as to allow lightissuing from the image intensifier tube to be transmitted to the imagesensor; non-binding filling which fills at least one gap which isbetween at least one pair of adjacent fiber optic plates among the atleast two fiber optic plates; and a detachable attaching mediumdetachably coupling between the image intensifier tube and the imagesensor.

According to the present invention there is also provided a method ofseparating an image intensifier tube detachably coupled to an imagesensor, comprising: a) providing an image intensifier tube detachablycoupled to an image sensor; and b) separating the image intensifier tubefrom the image sensor; wherein the separating does not substantiallydamage the image intensifier tube nor the image sensor.

According to the present invention there is further provided anapparatus for inspection of a sample, comprising: a radiation source,which is adapted to direct optical radiation onto an area of a surfaceof the sample; at least one image intensifier, each of which isdetachably coupled to an image sensor, so as to receive the radiationfrom the area over a certain angular range, and to provide intensifiedradiation to the image sensor; and at least one image sensor, each ofwhich is configured to receive radiation from at least one imageintensifier, so as to form at least one respective image of the area.

According to the present invention there is provided a method ofinspecting a sample, comprising: a) providing at least one imageintensifier tube detachably coupled to an image sensor with non-bindingfilling; b) directing optical radiation onto an area of a surface of asample to be inspected; c) receiving and intensifying the radiationscattered from the area using the at least one provided detachablycoupled image intensifier tube and image sensor, so as to form arespective images of the area, each of the provided detachably coupledimage intensifier tube and image sensor being configured to receive theradiation that is scattered to into a different, respective angularrange; and d) processing at least one of the respective images so as todetect a defect on the surface.

According to the present invention there is also provided a method ofinspecting a sample comprising: a) providing an image intensifier tubedetachably coupled to an image sensor; b) separating the imageintensifier tube from the image sensor; c) coupling at least one of theseparated image intensifier tube and image sensor in a combination ofimage intensifier tube and image sensor; d) directing optical radiationonto an area of a surface of a sample to be inspected; e) receiving theradiation scattered from the area using the combination coupled in (c)so as to form a respective image of the area; and f) processing theimage so as to detect a defect on the surface.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may be carriedout in practice, a preferred embodiment will now be described, by way ofnon-limiting example only, with reference to the accompanying drawings,in which:

FIG. 1A is a schematic, pictorial illustration of an image intensifierdetachably coupled to an image sensor, according to an embodiment of thepresent invention;

FIG. 1B is a schematic pictorial illustration of a magnetically focusedimage intensifier detachably coupled to an image sensor, according to anembodiment of the present invention;

FIG. 2 is a block diagram that schematically illustrates a system foroptical inspection, according to an embodiment of the present invention;and

FIG. 3 is a schematic side view of an optical collection moduleincluding image intensifiers detachably coupled to image sensors,according to an embodiment of the present invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Described herein are embodiments of the present invention relating to animage intensifier and image sensor which are detachably coupledtogether, and to methods and systems using such a combination.

As used herein, the phrase “for example,” “such as” and variants thereofdescribing exemplary implementations of the present invention areexemplary in nature and not limiting.

For example, when an image intensifier tube and image sensor arepermanently coupled together, the combination can no longer be used oncethe image intensifier tube and/or image sensor fails (assuming forexample, that the failed part cannot be repaired or cannot be partiallyreplaced saving the other part while coupled together). However, if theimage intensifier tube and image sensor are detachably coupled together,the image intensifier and image sensor can possibly be detached. Forexample, say the image intensifier tube fails, the image sensor may bestill be used, for example by detaching the failed image intensifiertube from the image sensor and coupling a replacement image intensifiertube to the detached image sensor. Although in this example it isassumed that the image intensifier tube fails, the converse may in somecases be true and when an image sensor fails, the failed image sensormay be detached from the image intensifier and a replacement imagesensor may be coupled to the detached image intensifier tube. Theexample of detaching the detachably coupled together image intensifiertube and image sensor in order to replace a failed element should beunderstood to be a non-limiting example and detachment for any reason iswithin the scope of the invention. For example, the image intensifiertube and image sensor may be detached in order to clean, repair,inspect, upgrade, etc. one or the other of the elements. In some cases,the detached image intensifier tube and image sensor may be re-coupledtogether instead of to replacement image sensor or image intensifiertube respectively. In some cases, the detached image intensifier may beused independently of any image sensor and/or the detached image sensormay be used independently of any image intensifier, for example thedetached image intensifier may be used in front of a photomultipliertube PMT.

Typically although not necessarily, the image sensor is comprised in acamera which also comprises necessary electronics. Therefore it may insome cases be more technically accurate to describe the imageintensifier as being detachably coupled to the camera. However since theterm camera does not always have a uniform meaning in the art, forclarity of description the terminology used below describes the imagesensor as being detachably coupled to the image intensifier or refers toa detachable combination of image intensifier tube and image sensor.

Hereinbelow, in order to improve readability, the abbreviation “DIIS” isused for Detachable combination of image Intensifier tube and ImageSensor.

FIG. 1A is a schematic view of a DIIS 10 comprising an image intensifier16 detachably coupled to an image sensor 34, according to an embodimentof the present invention.

In the illustrated embodiment, image intensifier tube 16 includes aphotocathode unit 14, for example a multialkali photocathode layer on aglass substrate and a screen unit 20, for example a phosphor layerstructure on a glass (fiber optic plate) substrate. Any other suitablephotocathode and/or screen unit may be used instead.

Depending on the embodiment, image intensifier 16 may be an imageintensifier of any generation and/or using any focusing method, asappropriate. As is known in the art, there are several known generationsof image intensifiers: The so-called “first generation imageintensifiers” are intensifier diodes that utilize only a singlepotential difference to accelerate electrons from the cathode to theanode (screen). The “second generation image intensifiers” utilizeelectron multipliers, i.e., not only the energy but also the number ofelectrons between input and output is significantly increased.Multiplication is achieved by use of a device called a microchannelplate (MCP), i.e. a thin plate of conductive glass containing many smallholes. In these holes, secondary electron emission occurs which leads tomultiplication factors of up to four orders of magnitude. The “thirdgeneration image intensifiers” employ MCP intensifiers withGallium-Arsenide photocathodes (instead of multialkali photocathodes asCs, Sb, K, Na, etc. normally used in first and second generationintensifiers or instead of bi-alkali or solar blind (CsTs) sometimesused in first or second generation intensifiers) to increase a luminoussensitivity of approximately 1,200 μA/lm instead of 300 μA/lm found inthe multialkali photocathodes. These GaAs photocathodes are also muchmore sensitive in the NIR region of the light spectrum. Modified thirdgeneration image intensifiers which are filmless (i.e. without an ionbarrier film) are sometimes termed “fourth generation imageintensifiers” or may be grouped under the term “third generation imageintensifiers”.

In these intensifiers, focusing is achieved by any of three approaches.The first approach includes placing the screen in close proximity to thephotocathode (proximity focus image intensifier). In the secondelectrostatic approach, electrodes focus electrons originating from thephotocathode onto the screen (electrostatic image intensifier orinverter image intensifier). In the third magnetic focus approach, amagnetic field parallel to the optical axis causes electrons to completeexactly one (or complete multiplication of one) full turn (magneticallyfocused image intensifier).

In some cases, image intensifier 16 may have more than one focusing, forexample if an MCP is used, and the more than one focusing may all usethe same focusing approach or may use a plurality of different focusingapproaches.

Depending on the embodiment, image sensor 34 can be any suitable solidstate optical image sensor. Examples of possible solid state opticalimage sensors which can be used as image sensor 34 include inter-alia: acharge coupled device (CCD) or a complementary-symmetry metal-oxidesemiconductor (CMOS) device.

As illustrated in FIG. 1A, a first fiber optic plate 22 is connected toa photo-emitting output area (back face) of screen unit 20. A secondfiber optic plate 32 connects to a photosensitive input area (frontface) of image sensor 34. In one embodiment each fiber optic plate 22and 32 is less than 4 fringes (surface quality). In one embodiment, oneor both of fiber optic plates 22 and 32 have a mechanism for reducedcrosstalk (for example fiber optic plate with extra mural absorptionEMA).

First fiber optic plate 22 and second fiber optic plate 32 are aligned(put into the correct relative position) so that the light issuing fromscreen unit 20 is transmitted to the photosensitive input surface ofimage sensor 34.

Typically, there is a small gap between first fiber optic plate 22 andsecond fiber optic plate 32. In one embodiment, the gap is about 0 to 5microns. Therefore, a non-binding filling 40 is used to fill the gapbetween the two plates 22 and 34. It should be understood by the reader,that the term “non-binding” filling 40 refers to a filling which allowsthe two fiber optic plates 22 and 32 to be separated from one another(and therefore image intensifier tube 16 and image sensor 34 to beseparated from one another) without substantially damaging the DIIS 10(for example without substantially damaging any of image sensor 34,image intensifier 16 or fiber optic plates 22 and 32).

Non-binding filling 40 has an index of refraction which is closer to theindex of refraction of fiber optic plates 22 and 34, than the index ofrefraction of air is to the index of refraction of the fiber opticplates, thereby preventing or minimizing Fresnel reflections. (Withoutnon-binding filling 40, Fresnel reflections would probably occur at theinterface between the fiber optic plate and air due to the differentrefractive indices). For example, non-binding filling 40 may have anindex of refraction similar to fiber optic plates 22 and 32. Continuingwith the example, the index of refraction of non-binding filling 40 maybe about 1.8. In another example, the index of refraction of non-bindingfilling 40 is not an identical match to that of the fiber optic plates.Continuing with the example, the index of non-binding filling 40 may beabout 1.5.

Non-binding filling 40 can be for example a gel or an oil. In oneembodiment, non-binding filling 40 has minimal outgassing. The size ofthe gap and the index value stated above are provided solely for furtherillustration to the reader and should not be construed as limiting.

A detachable attaching medium is used to detachably attach imageintensifier tube 16 to image sensor 34. The reader should understandthat the term “detachable” refers to an attaching medium which will stopattaching when separation of image intensifier 16 and image sensor 34from one another is desired. For example, the detachable attachingmedium can be removed, released, counteracted, etc. when separation ofimage intensifier 16 and image sensor 34 from one another is desired.Due to the usage of the detachable attaching medium, the attachment ofimage sensor 16 and image intensifier 34 as well as the separation ofimage sensor 16 and image intensifier 34 can be achieved withoutsubstantially damaging DIIS 10 (for example without substantiallydamaging any of image sensor 34, image intensifier 16 or fiber opticplates 22 and 32).

In one embodiment the detachable attaching medium at least includes anelastic material which allows fiber optic plates 22 and 32 to be pushedclose together (for example, when attaching image intensifier 16 andimage sensor 34 to one another) without substantially damaging DIIS 10(for example without substantially damaging any of image sensor 16,image intensifier 34, and/or fiber optic plates 22 and 32). Examples ofelastic material include inter-alia: spring(s), sponge(s), rubber, etc.

In the illustrated embodiment of FIG. 1A, detachable attaching medium 50includes: spring(s) 56, screws 51, 53, and 58, and mechanical parts 52and 54. One or more springs 56 in conjunction with one or more screws 58are used to connect mechanical part 52 to mechanical part 54. Mechanicalpart 52 is shown connected to a camera 70 comprising image sensor 34 andmechanical part 54 is shown connected to image intensifier 14. Althoughmechanical part 52 is shown directly and detachably connected to camera70 with one or more screws 51, in other embodiments mechanical part 52can be indirectly and/or permanently attached to camera 70. In otherembodiments, mechanical part 52 can be directly or indirectly attached,either detachably or non-detachably to image sensor 34. Althoughmechanical part 54 is shown detachably and directly connected to imageintensifier 16 with one or more screws 53, in other embodiments,mechanical part 54 may be indirectly and/or permanently attached toimage intensifier tube 16. In other embodiments mechanical part 52 maybe omitted and for example, one or more springs in conjunction with oneor more screws may connect for example between camera 70 (or imagesensor 34) and image intensifier 16, or for example between camera 70(or image sensor 34) and mechanical part 54. In other embodiments,mechanical part 54 may be omitted and for example, one or more springsin conjunction with one or more screws may connect for example betweencamera 70 (or image sensor 34) and image intensifier 16, or for examplebetween mechanical part 52 and image intensifier 16. In anotherembodiment, detachable attachable medium 50 can include screws andsprings, with camera 70 held detachably and firmly (for example withscrews) to image intensifier tube 16, and inside camera 70, image sensor34 “floats” on springs.

In one embodiment, first fiber optic plate 22 and image intensifier 16are commercially available as one unit and/or permanently coupledtogether and are thus shown in FIG. 1A. In one embodiment, second fiberoptic plate 32, camera 70 and image sensor 34 are commercially availableas one unit and/or permanently coupled together and are thus shown inFIG. 1A. However it should be evident that in other embodiments, some orall of these elements may be detachably coupled to one another. Forexample, in one embodiment, image sensor 34 may be detachably coupled tocamera 70.

It should be evident that FIG. 1A illustrates only one example ofpossible detachable attaching medium. In other embodiments, thedetachable attaching medium may comprise less than all of elements 51,52, 53, 54, 56, and 58. In other embodiments, the detachable attachingmedium may comprise additional elements in addition to elements 51, 52,53, 54, 56, and 58. In other embodiments, the detachable attachingmedium may comprise elements different than some or all of elements 51,52, 53, 54, 56, and 58. In other embodiments, the functionality providedby elements 51, 52, 53, 54, 56, and 58 may be distributed differentlyamong those elements.

Although as mentioned above image the image intensifier in the DIIS mayuse any focusing approach, in some applications it may be advantageousto have an image intensifier which is magnetically focused. For example,in some cases the usage of a magnetically focused image intensifier in aparticular application provides superior optical performance and/or thepossibility of enhancing the lifetime of the image intensifier (comparedto a proximity focused or electrostatic focused image intensifier). Insome of these cases the superior optical performance includes any of thefollowing inter-alia: better resolution, lower halo, and the possibilityof having a potential difference greater than 10 to 15 KV in the imageintensifier and therefore a higher gain.

For the sake of further illustration to the reader, FIG. 1B illustratesa DIIS which comprises a magnetically focused image intensifieraccording to an embodiment of the present invention. For simplicity'ssake FIG. 1B replicates the DIIS illustrated in FIG. 1A, however alsoillustrates is a magnet 60 which surrounds image intensifier 16. Amagnetic field produced by a magnet 60, parallel to the optical axis,causes electrons to complete exactly one full turn.

Magnet 60 is shown detachably attached to mechanical part 52 with one ormore screws 62 in FIG. 1B. In other embodiments magnet 60 can be placedaround image intensifier 14 using a different technique.

The illustration of image intensifier 16 as magnetically focused in FIG.1B should not be construed as binding. The image intensifier in a DIISof this invention may use any focusing approach, which may varydepending on the embodiment.

The combination of elements in a DIIS may vary depending on theembodiment and is not limited to the combination of elements illustratedin FIG. 1A or 1B. At a minimum, a DIIS comprises an image sensor and animage intensifier in a detachable combination. However, other elementsshown in FIG. 1A or 1B may in some embodiments be omitted from a DIIS.In some embodiments, additional elements not shown in FIGS. 1A and 1Bmay be included in a DIIS.

In some embodiments, the image intensifier tube and the image sensor inthe DIIS are later detached from one another. For example, assume animage intensifier tube and an image sensor have been previouslydetachably connected together into a DIIS, for example as illustrated inFIG. 1A. Assuming the configuration illustrated in FIG. 1A, at a laterpoint in time, image intensifier tube 16 and first fiber optic plate 22may be detached from image sensor 34 and second fiber optic plate 32,without substantially damaging DIIS 10 (for example withoutsubstantially damaging any of image sensor 16, image intensifier 34,and/or fiber optic plates 22 and 32). For example, at least screw(s) 58may be unscrewed, releasing the connection between mechanical parts 52and 54. Optionally, depending on the embodiment, any of the otherscrew(s) 51, 53 may also be unscrewed, and/or any of elements in FIG. 1Awhich are detachable (for example, any of elements 51, 52, 53, 54, 56,and 58) may be removed.

Once detached, the image intensifier tube and/or image sensor may beretained as is, processed as is (for example inspected), modified (forexample repaired, upgraded, or cleaned) or discarded. Optionally, thedetached image intensifier tube and/or image sensor may be re-attachedtogether, or one or both may be reattached to another element (e.g. torespectively another image sensor or image intensifier tube, or to adifferent device). For example, in one embodiment, detachment may occurupon the failure or degradation of the image intensifier tube and thedetached image sensor may later be attached to another image intensifiertube, either detachably as described above or permanently. It is alsopossible that one or both of the detached image intensifier tube and/orimage sensor may not be later attached to any other element.

In one embodiment, the non-binding filling which filled the gap betweenthe two fiber optic plates corresponding respectively to the imagesensor and image intensifier, is removed any time after detaching theimage intensifier tube and image sensor from one another. In thisembodiment, if the detached image intensifier and/or image sensor islater detachably attached to each other or to another element, newnon-binding filling is applied if necessary. However, in anotherembodiment, the non-binding filling is not necessarily removed and mayoptionally be reused when later detachably attaching one or both of thedetached image intensifier and/or the image sensor to each other or toanother element.

Applications which incorporate one or more DIIS in accordance withembodiments described above are not limited by the invention. Forfurther illumination to the reader, however, it will now be described anapplication incorporating an embodiment of a DIIS, namely a dark fieldinspection system for inspecting wafers as described in co-pending andco-assigned U.S. Ser. No. 10/511,092 (U.S. published application number20050219518), said application hereby incorporated by reference hereinFIG. 2 is a block diagram that schematically illustrates a system 220for optical inspection of a semiconductor wafer 222, in accordance withan embodiment of the present invention. Typically, wafer 222 ispatterned, using methods of semiconductor device production known in theart, and system 220 applies dark-field optical techniques to detectdefects on the surface of the wafer. Alternatively, however, theprinciples embodied in system 220 may be applied to unpatterned wafersand to inspection of other types of samples and surfaces as well, suchas masks and reticles. Furthermore, although system 220 is dedicated todark-field inspection, aspects of the present invention may also beapplied in bright-field inspection, as well as in other areas ofillumination, inspection and imaging.

System 220 comprises an illumination module 224, which illuminates thesurface of sample 222 using pulsed laser radiation. Typically, module224 is able to emit the laser radiation selectably at two or moredifferent wavelengths, either simultaneously or one at a time. The laserradiation at any of the laser wavelengths may be directed by module 224to impinge on wafer 222 either along a normal to the wafer surface orobliquely, as described in further detail in US Published ApplicationNumber 20050219518. The illumination module may be configured to emitoptical radiation at wavelengths in the visible, ultraviolet (UV) and/orinfrared (IR) ranges. The terms “illumination” and “optical radiation”as used herein should therefore be understood as referring to any or allof the visible, UV and IR ranges.

The radiation scattered from wafer 222 is collected over a large rangeof angles by an optical collection module 226. Module 226 comprisescollection optics 228, which image the surface of wafer 222 ontomultiple DIIS 230. Optics 228 may comprise either a single objectivewith high numerical aperture (NA) or a collection of individualobjectives, one for each DIIS 230. Details of both of these alternativeoptical configurations are described in further detail in US PublishedApplication Number 20050219518, and details of DIIS 230 are describedhereinbelow. Optics 228 and DIIS 230 are arranged so that all the DIISimage the same area on the wafer surface, i.e., the area illuminated byillumination module 224, while each DIIS 230 captures the radiation thatis scattered into a different angular range. Each DIIS 230 includes atwo-dimensional array of detector elements, such as a CCD or CMOS array,as is known in the art. Each detector element of each of the arrays isimaged onto a corresponding spot within the area irradiated byillumination module 224. Thus, the scattering characteristics of anygiven spot on wafer 222 as a function of angle can be determined basedon the signals generated by the corresponding detector elements in thedifferent DIIS 230.

DIIS 230 are typically synchronized with the laser pulses fromillumination module by a system controller 232, so that each imageoutput frame generated by each DIIS 230 corresponds to the radiationscattered from a single laser pulse. The output from each DIIS 230 isreceived, digitized and analyzed by an image processor 234. The imageprocessor which is described in further detail in U.S. PublishedApplication Number 20050219518, typically comprises dedicated hardwaresignal processing circuits and/or programmable digital signal processors(DSPs). A mechanical scanner, such as an X-Y-Z stage 236 translateswafer 222, typically in a raster pattern, so that each laser pulse fromillumination module 224 irradiates a different area of the surface ofthe wafer, adjacent to (and typically slightly overlapping with) thearea irradiated by the preceding pulse. Alternatively or additionally,the illumination and collection modules may be scanned relative to thewafer.

Image processor 234, processes each of the image frames that is outputby each DIIS 230 in order to extract image features that may beindicative of defects on the wafer surface. The image features arepassed to a host computer 238, typically a general-purpose computerworkstation with suitable software, which analyzes the features in orderto generate a defect list (or defect map) with respect to the waferunder inspection.

The area irradiated by module 224 and imaged by DIIS 230 can be scannedusing stage 236 over the entire wafer surface, or over a selected areaof the surface. If the pulses emitted by module 224 are sufficientlyshort, substantially less than 1 μs, for example, stage 236 maytranslate wafer 222 continuously in this manner without causingsignificant blur in the images captured by the DIIS. The irradiated areatypically has dimensions on the order of 2×1 mm, although the area canbe enlarged or reduced using magnification optics in the illuminationmodule as described in more detail in U.S. Published Application Number20050219518. Assuming each DIIS 230 includes an array of about 2000×1000detector elements, the size of each pixel projected onto the wafersurface is then roughly 1×1 μm. With module 224 operating at arepetition rate of 400 pulses/sec, the data output rate of each DIIS 230to image processor 234 will be 800 Mpixels/sec. At this rate, forinstance, an entire 12″ semiconductor wafer can be scanned at 1 μmresolution in less than 2 min. It will be understood, however, thatthese typical figures of image resolution, size and speed are citedsolely by way of example, and larger or smaller figures may be useddepending on system speed and resolution requirements.

Controller 232 also adjusts the Z-position (height) of stage 236 inorder to maintain the proper focus of DIIS 230 on the wafer surface.Alternatively or additionally, the controller may adjust the DIIS opticsfor this purpose. Further alternatively or additionally, the controllermay instruct image processor 234 and host computer 238 to correct fordeviations in the scale and registration of the images captured bydifferent DIIS 230 so as to compensate for height variations.

In order to verify and adjust the focus, controller 232 uses anauto-focus illuminator 240 and an auto-focus sensor module 242.Illuminator 240 typically comprises a laser (not shown), such as a CWdiode laser, which emits a collimated beam at an oblique angle onto oradjacent to the area of the surface of wafer 222 that is illuminated byillumination module 224, forming a spot on the wafer surface. Variationsin the Z-position of wafer 222 relative to collection module 226 willthen result in transverse displacement of the spot. Sensor module 242typically comprises a detector array (also not shown), which captures animage of the spot on the wafer surface. The image of the spot isanalyzed in order to detect the transverse position of the spot, whichprovides controller 32 with a measurement of the Z-position of the wafersurface relative to the collection module. The controller may drivestage 236 until the spot is in a pre-calibrated reference position,indicative of proper focus.

The beam emitted by illuminator 240 may pass through collection optics228 on its way to the wafer surface, and sensor module 242 may likewisecapture the image of the spot on the surface through the collectionoptics. In this case, illuminator 240 preferably operates in a differentwavelength range from illumination module 224. Thus, appropriate filtersmay be used to block scatter of the auto-focus beam into DIIS 230, aswell as preventing interference of the pulsed beam from module 224 withthe auto-focus measurement.

Alternatively, other means of auto-focus detection may be used, as areknown in the art. For example, a capacitive sensor may be used todetermine and adjust the vertical distance between the optics and thewafer surface.

FIG. 3 is a schematic side view of collection module 226, in accordancewith an embodiment of the present invention. In this embodiment and inthe embodiment shown in FIG. 2, module 226 is shown as comprising fiveDIIS 230. Alternatively, module 226 may comprise a smaller or greaternumber of DIIS, typically as many as ten DIIS. As noted above, all theDIIS image scattered radiation from a common area 348 on the surface ofwafer 222, but each DIIS is configured to collect the radiation along adifferent angular axis (i.e., a different elevation and/or azimuth).Although system 220 is designed mainly for use in dark-field detection,one or more of DIIS 230 may be used for bright-field detection, as well,in conjunction with either the normal-incidence or oblique-incidenceillumination beam.

An objective 350 collects and collimates the scattered light from area348. In order to collect scattered light at low elevation, objective 350preferably has a high NA, most preferably as high as 0.95. An exemplarydesign of objective 350, using multiple refractive elements, isdescribed further in U.S. Published Application Number 20050219518.Alternatively, objective 350 may comprise a reflective or catadioptricelement, as described, for example, in U.S. Pat. No. 6,392,793 to Chuanget al, which is hereby incorporated by reference herein. Each of DIIS230 is positioned, as shown in FIG. 3, to receive a particular angularportion of the light collected by objective 350.

For each DIIS 230, a bandpass filter 352 selects the wavelength rangethat the DIIS is to receive. Typically, filter 352 selects one of thetwo wavelengths emitted by illumination module 224, while rejecting theother wavelength. Filter 352 may also be implemented as a dichroicbeamsplitter, and configured so that one DIIS 230 receives the scatteredlight along a given angle at one wavelength, while another DIIS receivesthe scattered light along the same angle at the other wavelength. As afurther alternative, filter 352 may be chosen to pass radiation inanother wavelength range, such as a band in which wafer 222 is expectedto fluoresce. For example, when organic materials, such as photoresist,are irradiated at 266 nm, they tend to fluoresce in the range of 400 nm.Thus, setting filter 152 to pass light in the 400 nm band allows DIIS230 to detect defects in the organic material or residues thereof.

A spatial filter 354 can be used to limit the collection angle of eachDIIS 230, by blocking certain regions of the collimated scattered light.The spatial filter is especially useful in eliminating backgrounddiffraction from repetitive features on patterned wafers. The spatialfilter is chosen, based on the known diffraction pattern of the featureson the wafer surface, to block these strong diffraction nodes, in orderto enhance the sensitivity of system 220 to actual defects, as is knownin the art. This use of spatial filtering for this purpose is described,for example, in U.S. Pat. No. 6,686,602 to Some, whose disclosure isincorporated herein by reference. This patent describes a method forcreating spatial filters adaptively, in response to the diffractionlobes of different sorts of wafer patterns. This method may beimplemented in filters 354 in module 226. Alternatively, spatial filters354 may comprise fixed patterns, as is known in the art.

A rotatable polarizer 356 is provided in the optical path in order toselect the direction of polarization of scattered light that is to bereceived by DIIS 230. The polarizer is useful, for example, in improvingdetection sensitivity by rejecting background scatter due to roughand/or highly-reflective surface structures on wafer 222. Optionally,polarizer 356 is implemented as a polarizing beamsplitter, which isconfigured so that two DIIS 230 receive the light scattered along agiven angle in orthogonal polarizations.

As a further option (not shown in the figures), the optical path maycomprise a beamsplitter, which divides the light scattered along a givencollection angle between two or more different DIIS 230. Thebeamsplitter may be used for wavelength division, as mentioned above, orto divide the same wavelength between the two or more DIIS in apredetermined proportionality. Different spatial filters 354 may be usedfollowing the beamsplitter in the beam paths to the different DIIS, inorder to filter out diffraction lobes due to different sorts of patternson the wafer. As a further alternative, the beamsplitter may divide thelight scattered along a given angle unequally between two or more of theDIIS, for example, in a ratio of 100:1. This arrangement effectivelyincreases the dynamic range of system 220, since the DIIS receiving thesmaller share of the radiation is still able to generate meaningfulimage data even in areas of bright scatter, in which the DIIS receivingthe larger share of the radiation is saturated. An arrangement of thissort is described, for example, in U.S. Pat. No. 6,657,714 to Almogy etal whose disclosure is incorporated herein by reference.

A focusing lens 358 focuses the collected and filtered light onto DIIS230. Lens 358 may be adjustable, either manually or under motorizedcontrol. A variable magnifier 360 may be used to change the size of themagnified image received by the DIIS. Alternatively, the functions oflens 358 and magnifier 360 may be combined within a single optical unitfor each DIIS. The magnifier determines the resolution of the imagecaptured by DIIS 230, i.e., the size of the area on the wafer surfacethat corresponds to each pixel in the output image from the DIIS.Magnifier 360 is typically operated in conjunction with telescopes inillumination module 224, so that size of the illuminated area is roughlyequal to the area imaged by the DIIS.

Each DIIS 230 comprises an image intensifier 362, whose photocathode isaligned at the image plane of the focusing lens 358 and magnifier 360.Any suitable type of image intensifier tube of any generation/focusingapproach(es) may be used for this purpose. For the sake of furtherillustration to the reader, non-limiting examples include first andsecond generation image intensifiers such as the C6654 image intensifierproduced by Hamamatsu Photonics K.K. (Shizuoka-ken, Japan) or firstgeneration magnetically focused image intensifiers produced by PhotekLtd (East Sussex, UK). To provide optimal imaging in the demandingenvironment of system 220, intensifier 362 preferably has high bandwidthand high resolution, and is preferably capable of gated operation, withhigh current and low phosphor memory, at the repetition rate of laserhead 50—typically up to about 1000 pulses per sec. In one embodiment,the useful diameter of intensifier 362 is preferably at least 18 mm. Inanother embodiment, a larger diameter of intensifier 362, in the rangeof 25-40 mm, is used.

Although as mentioned above, focusing in intensifier 362 may be achievedby any approach (proximity, electrostatic, magnetic), in one embodiment,intensifier 362 is magnetically focused and results in superior opticalperformance and/or the possibility of enhancing the lifetime, asdescribed above.

The output of image intensifier 362 is focused by optics 364 onto animage sensor 366. The optics 364 comprises, two fiber optic plates witha non-binding filling in the gap between the two plates as illustratedand described above with reference to FIGS. 1A and 1B. Image sensor 366comprises a two-dimensional matrix of detector elements, such as a CCDor CMOS array, as is known in the art. For example, the image sensor maycomprise a CMOS digital image sensor, such as model MI-MV13, made byMicron Technology Inc. (Boise, Id.). This sensor has 1280×1024 pixels,with 12 μm vertical and horizontal pitch, and a frame rate up to 500frames per second for full frames. A detachable attaching medium is usedto attach image intensifier 362 to image sensor 366 in DIIS 230, asillustrated and described above with reference to FIGS. 1A and 1A.

The use of image intensifiers 362 increases the sensitivitysubstantially compared to using image sensors 366 alone withoutintensification. Image intensifiers 362 intensifiers may be gated, insynchronization with the light pulses from illumination module 224, inorder to increase the sensitivity of the DIIS and reduce their noiselevels still further. Typically, the photocathodes of intensifiers 362are chosen to have high quantum efficiency at the wavelengths emitted bythe illumination module 224, while the phosphors of the intensifiers 362may be chosen to emit light in a different wavelength range in whichimage sensors 366 have high responsivity. Thus, the image intensifiers362, in addition to amplifying the incident scattered light, are alsouseful in downconverting the ultraviolet (UV) and blue light that isscattered from wafer 222 to the green or red range, to which the siliconimage sensors are more responsive. In addition, intensifiers 362 act aslow-pass spatial filters, and may thus help to smooth high-frequencystructures in the scattered light that might otherwise cause aliasing inthe images output by sensors 366.

Intensifiers 362 preferably have high resolution, as dictated by theresolution of sensors 366. For example, to take full advantage of theresolution of the above-mentioned MV13 sensor, intensifier 362 should bedesigned to provide 1640 distinct pixels along the image diagonal. Thisresolution criterion may also be expressed in terms of the modulationtransfer function (MTF) of the intensifier, giving for example MTF=30%for a test image with 33 line pairs/mm or 30%-40% at 40 line pairs/mmdepending on the embodiment of intensifiers 362. Bright points in theimage captured by DIIS 230 can result in formation of a bright halo,generally due to reflections inside the image intensifier tube, whichmay compromise the resolution of the image. Intensifiers 362 arepreferably designed to suppress such reflections so that the halodiameter is no more than 0.2 mm in any case. Furthermore, in order toexploit the full range of sensitivity of sensor 366, intensifier 362should exhibit linear behavior up to high maximum output brightness(MOB), typically on the order of 600 μW/cm².

For brevity of description, other details relating to system 220 asprovided in U.S. published application number 20050219518 are herebyincorporated by reference rather than being replicated here.

For ease of understanding, the description above described a singlefiber optic plate 22 coupled to a photo-emitting output area of imageintensifier tube 16 (or 362) and a single fiber optic plate 32 coupledto a photosensitive input area of image sensor 34 (or 366) with a gapbetween the two plates filled with a non-binding filling 40. However, itshould be evident to the reader that in some embodiments single fiberoptic plate 22 may be replaced with a plurality of fiber optic plates 22and/or single fiber optic plate 32 may be replaced with a plurality offiber optic plates 32. In addition, or instead there may be fiber opticplates in between image intensifier tube 16 (or 362) and image sensor 34(or 366) which are not clearly associated with either image intensifier16 (362) or image sensor 34 (366). Therefore in order to understand thepossible embodiments, the reader should recognize that there are atleast two fiber optic plates between the photo-emitting output area ofimage intensifier tubes 16 (362) and the photosensitive input area ofimage intensifier 34 (366). (For ease of explanation, each fiber opticplate is designated 22/32 because the association or non-association ofeach plate with image intensifier tube 16 (362) or image sensor 34 (366)may vary with the embodiment). Depending on whether a particular fiberoptic plate 22/32 has a neighboring fiber optic plate 22/32 on one sideor both sides, that fiber optic plate 22/32 can be considered to belongto one or two pairs of adjacent fiber optic plates, respectively. Thegap between one pair of adjacent fiber optic plates 22/32 is filled withnon-binding filling 40, but the selection of which pair of adjacentplates 22/32 out of all possible pairs of adjacent plates 22/32 has thegap filled with non-binding filling 40 may vary depending on theembodiment. Also depending on the embodiment, if there are other pairsof adjacent fiber optic plates 22/32, the gap(s) between all otherpair(s) of adjacent fiber optic plates 22/32 may be filled withnon-binding filling 40, the gap(s) between all other pair(s) of adjacentfiber optic plates 22/32 may be filled with a known in the art adhesive,or some of the gap(s) between the other pair(s) of adjacent fiber opticplates 22/32 may be filled with non-binding filling 40 whereas thegap(s) between other(s) of the other pair(s) of adjacent fiber opticplates 22/32 may be filled with a known in the art adhesive. The readerwill recognize that as long as the gap between at least one pair ofadjacent fiber optic plates 22/32 is filled with non-binding filling 40,those pair(s) of adjacent fiber optic plates 22/32 may be separated fromone another (and therefore image intensifier tube 16 (362) and imagesensor 34 (366) may be separated from one another) without substantiallydamaging the DIIS 10 (for example without substantially damaging any ofimage sensor 34 (366), image intensifier 16 (362) or fiber optic plates22/32).

The methods and systems described above, apply to embodiments with morethan two fiber optic plates 22/32 between the photo-emitting output areaof image intensifier tubes 16 (362) and the photosensitive input area ofimage intensifier 34 (366), mutatis mutandis. An example is now providedwhich uses more than two fiber optic plates 22/32. The number of plates,types of bindings between the plates, and other assumptions of theexample are provided solely for further illustration to the reader andshould therefore not be construed as limiting. It is assumed that afirst fiber optic plate is coupled to image intensifier 16 (362) and asecond fiber optic plate is attached with an adhesive to the first fiberoptic plate. It is further assumed that a third fiber optic plate iscoupled to image sensor 34 (366) and a fourth fiber optic plate isaligned between third fiber optic plate and second fiber optic plate. Itis further assumed that a non-binding filling 40 fills the gap betweenthe pair of third fiber optic plate and fourth fiber optic plate and anon-binding filling 40 (not necessarily the same filling) fills the gapbetween the pair of second fiber optic plate and fourth fiber opticplate. In this example, when detachably attaching image sensor 34 (366to image intensifier 16 (362) using a detachable attaching medium asdescribed above, it is assumed that at least the pair of third fiberoptic plate and fourth optic plate are pushed close together and thepair of second fiber optic plate and fourth fiber optic plate are pushedclose together. In one embodiment of the example, the detachableattaching medium is assumed to include an elastic material which atleast allows the pair of third and fourth fiber optic plates to bepushed close together and the pair of second and fourth fiber opticplates to be pushed close together without substantially damaging any ofthe fiber optic plates (for example first, second, third and fourthfiber optic plates), image sensor 34 (366) and/or image intensifier 16(362). In this example if detachment of image sensor 34 (366) from imageintensifier 16 (362) is later desired as described above, then in someembodiments the detachment process may include inter-alia the separationof second and fourth fiber optic from one another and/or the separationof third and fourth fiber optic plates from one another.

While the invention has been shown and described with respect toparticular embodiments, it is not thus limited. Numerous modifications,changes and improvements within the scope of the invention will nowoccur to the reader.

1. An apparatus for inspection of a sample, comprising: a radiationsource, which is adapted to direct optical radiation onto an area of asurface of the sample; at least one image intensifier, each of which isdetachably coupled to an image sensor, so as to receive the radiationfrom the area over a certain angular range, and to provide intensifiedradiation to the image sensor; and at least one image sensor, each ofwhich is configured to receive radiation from at least one imageintensifier, so as to form at least one respective image of the area. 2.The apparatus of claim 1, further comprising: an image processor, whichis adapted to process at least one of the respective images so as todetect a defect on the surface.
 3. The apparatus of claim 1, furthercomprising at least two fiber optic plates for each image intensifier,wherein said image intensifier is further detachably coupled to one ofthe image sensors using said at least two fiber optic plates, andwherein non-binding filling fills at least one gap which is between atleast one pair of adjacent fiber optic plates among said at least twofiber optic plates.
 4. The apparatus of claim 3, wherein saidnon-binding filling is an oil or gel with an index of refraction that iscloser to an index of refraction of said fiber optic plates than anindex of refraction of air is to said index of refraction of said fiberoptic plates.
 5. The apparatus of claim 3, further comprising: adetachable attaching medium detachably coupling each said imageintensifier to one of the image sensors, wherein said medium includes anelastic material configured to at least allow said at least one pair ofadjacent fiber optic plates to be pushed close together withoutsubstantially damaging any of said image intensifier tube, image sensor,and said fiber optic plates.
 6. The apparatus of claim 1, furthercomprising: a detachable attaching medium detachably coupling each saidimage intensifier to one of the image sensors.
 7. The apparatus of claim6, wherein said detachable attaching medium includes at least one screwconfigured when screwed in to prevent separation of each said imageintensifier tube from said one image sensor.
 8. The apparatus of claim1, wherein at least one of said image intensifiers is magneticallycoupled.
 9. A method of inspecting a sample, comprising: a. providing atleast one image intensifier tube detachably coupled to an image sensorwith non-binding filling; b. directing optical radiation onto an area ofa surface of a sample to be inspected; c. receiving and intensifying theradiation scattered from the area using said at least one provideddetachably coupled image intensifier tube and image sensor, so as toform a respective images of the area, each of said provided detachablycoupled image intensifier tube and image sensor being configured toreceive the radiation that is scattered to into a different, respectiveangular range; and d. processing at least one of the respective imagesso as to detect a defect on the surface.
 10. The method of claim 9,wherein said non-binding filling fills at least one gap which is betweenat least one pair of adjacent fiber optic plates among at least twofiber optic plates aligned between said image intensifier tube and saidimage sensor.
 11. A method of inspecting a sample comprising: a.providing an image intensifier tube detachably coupled to an imagesensor; b. separating said image intensifier tube from said imagesensor; c. coupling at least one of said separated image intensifiertube and image sensor in a combination of image intensifier tube andimage sensor; d. directing optical radiation onto an area of a surfaceof a sample to be inspected; e. receiving the radiation scattered fromthe area using said combination coupled in (c) so as to form arespective image of the area; and f. processing said image so as todetect a defect on the surface.
 12. The method of claim 11, wherein saidcoupling in (c) includes detachably coupling the same or different imageintensifier tube to said separated image sensor using at least two fiberoptic plates and a non-binding filling which fills at least one gapwhich is between at least one pair of adjacent fiber optic plates amongsaid at least two fiber optic plates.